1. Field of the Invention
The present invention relates to an automatic positioning system for aligning a target on a first object with a key on a second object. More specifically, the invention relates to a target that provides compensation for processing variations in generating the target.
2. Description of the Related Art
A plurality of masks are used in the manufacturing process of semiconductors and thin film magnetic transducers. Automatic positioning systems are used to align each mask with the substrate upon which the device is being manufactured. Such a positioning system is the Ultrateck STEPPER 2000/2700 and a description of how the positioning system actual works can be obtained from Ultrateck Inc., Attention Technical Training Documentation, 3250 Scott Blvd., Santa Clara, Calif. 95054.
Normally during the processing, a mask is used to place a target and a first pattern on the substrate. Subsequent masks contain a key and a pattern, where the pattern on the subsequent masks are to be located in a specified spacial relationship with regards to either the first pattern or prior patterns established by prior subsequent masks. The positioning system uses the target on the substrate and the key on each of the subsequent masks to establish the spacial relationship of a pattern on substrate to the patterns on the subsequent masks.
Referring to FIG. 1, an illustration of the relationship between the target 11 on the substrate 10 and the key 21 on mask 20 is shown. At present, the target 11 usually takes the form of a cross which is symmetrical about the center 13 of the orthogonal axes. The pattern, represented by the square 14 with center 16, was established on the substrate 10 at the same time as the target 11. On mask 20, a key 21, usually in the form of a cross approximately the same size as the cross of target 11, is used through which the substrate is monitored by a sensing mechanism within the positioning system. The pattern, represented by square 24 having center 26, on mask 20 is to be positioned in a spacial relationship with square 14 on substrate 10. It is normally desired to have the center 26 of square 24 aligned with the center 16 of square 14.
The positioning system first positions the substrate such that the scanning area of the positioning system will not experience the ends of the legs of cross 11. The positioning system will move the substrate the scanning distance first along a horizontal plane and the edges of the vertical legs of cross 11 will be detected through key 21 by the sensing mechanism whose output is a signal containing information as to the location of the two edges of the vertical legs of target 11 with relationship to key 21 which is used to determine the center of the target in the horizontal plane. The positioning system will next move the substrate the scanning distance along a vertical plane and the edges of the horizontal legs of cross 11 will be detected through key 21 by the sensing mechanism whose output is a signal containing information as to the location of the two edges of the horizontal legs of target 11 with relationship to the center 13 of key 21 which is used to determine the center of the target in the vertical plane. Once the system has determined the location of the center of target 11 with reference to the center 13 of key 21, the system can then move the substrate 10 such that the center 16 of square 14 matches the center 26 of square 24. By using the center of the target 11, the system is self compensating for process variation in the target 11 as long as the requirements are to match centers of the pattern on the substrate and the pattern on the mask.
This self correcting feature is exemplified by a target 12 (shown in dotted lines) and square 15 (shown in dotted lines). Here the target 12 was designed to be of the size of target 11 and the square 14 was designed to be the size of square 14. However, due to process variations the resulting size of target 11 and square 14 was smaller than the target 11 and square 14 on the mask used to lay down the target 11 and square 14. Since the target and the pattern are exposed to the same process variations, the change in target and pattern size will be the same. Even though the resultant size of target 12 and square 14 were smaller, the centers of target 12 and square 14 were not changed. Therefore the positioning system will find the same center for target 12 as it did for target 11 and will match the center 16 of target 15 with center 26 of square 24 on mask 20.
Since square 24 is the same size as square 14, it can be seen that although the center 16 of square 15 matches the center 26 of square 24 the edges of square 24 do not match the edges of square 15.
In some designs, specially in the manufacture of thin film magnetic transducers, it is critical to match an edge on the substrate to an edge on the mask. As can be seen from the forgoing description, process variations of the target can result in losing the relationship between the edges on the substrate and the edges of the mask.